The torque of internal combustion engines, in particular of diesel engines, is usually controlled in that a torque desired by the driver is ascertained, for example, via the actuation angle of a pedal travel sensor (accelerator pedal). A fuel mass to be supplied is ascertained as a function of the pedal value or the desired torque. For this purpose, normally a precontrol is first implemented by the use of stored characteristics maps. A subsequent control is implemented, for example, by measuring the inner cylinder pressure and by correcting the fuel quantity accordingly.
Since in normal operation, diesel engines are always operated at a very high amount of excess air, that is, at lambda>>1, normally no precise lambda control is required. In diesel engines, setting a lower lambda value (at λ≈1 or λ<1) and an appropriate control are only necessary in certain situations. Such situations exist, for example, when an NOx adsorption catalyst of the exhaust system is to be desulfurized or a particulate filter is to be regenerated. In order to produce a stoichiometric or hypostoichiometric air ratio required for these processes, it is convention for a method to supply the fuel by at least two injection processes such that, in addition to the main injection (or further injections), a post-injection takes place, which is implemented at a time at which the combustion occurs at low efficiency and thus in a manner only partly effective in terms of torque. The post-injection quantity supplied during the post-injection event, by contrast, combusts at only a low or even no torque-effectiveness, but produces the desired low air ratio by consuming the remaining oxygen in the combustion chamber. Overall, a main injection thus occurs, in which the supplied main injection quantity combusts in a torque-effective manner with excess air, and a post-injection, in which the supplied post-injection quantity combusts with only a limited torque-effectiveness while consuming oxygen and thus lowering the lambda value. The lambda feedback control occurs by measuring the oxygen content of the exhaust gas, for example, by using a lambda probe and by influencing the post-injection quantity accordingly. The lambda control is usually also first precontrolled.
The disadvantage of the conventional procedure is that the two controls affect each other disadvantageously. If the main injection quantity is influenced as a controlled variable for the torque control, then this has a certain effect on the resulting lambda value. On the other hand, influencing the post-injection quantity as a control mechanism for the lambda control has an—albeit small—effect on the resulting torque.